I personally am not into NTR (I love sharing though). But considering that this is the game about incest, NTR could bring some normality into everything and give D some more character. It's healthy for a young teenager to broaden her horizons and have some fun separately from her parent, maybe even get an actual caring boyfriend of her own age, at least to keep up appearance and not to attract too much attention. F and D are already too obvious and everybody is starting to get hints. Their luck can't be infinite, there gotta be some bad consequences for being too careless. And even if we ignore this, she might want to explore having a separate relationship when she finds that F wasn't faithful (no NTR on D only route). Maybe once she learns about Georgina and F, she doesn't immediately accept it. He can date all those pretty ladies behind her back, why can't she? I think turnabout is a fair play and wouldn't be surprised if she decides to have an affair with either of the photographers. Actually, it could also be a character arc for them. She might get too careless and eventually learn that photographer from Ch.1 is actually related to Lucas, she'd get scared and get back to F, saying that she was wrong and that F is perfect for her and she doesn't need anyone else because her luck with guys is horrible and only F is always there to protect her. Alternately, if she gets with Ryan, she might fall for him but then guiltily confess to F, tell him that she's confused and doesn't know what to do. There might be a choice to tell her that she shouldn't see anyone else (D remains exclusive with F and occasionally has fun with girls) or let her date Ryan (ends in open poly relationship with Georgina, D and Ryan).